1. Field of the Invention
The present invention relates to an infrared detector and a fabricating method thereof, in particular, an infrared detector that detects a temperature of an infrared receiver with a thermopile and a fabricating method of the infrared detector.
2. Description of the Related Art
Infrared ray is a collective term for an electromagnetic wave having a wavelength substantially from 0.72 to 1000 μm. It is categorized largely into a short wavelength band (3 to 5 μm) and a long wavelength band (8 to 14 μm). An infrared sensor is divided, from the viewpoint of the operation principle, into a quantum type and a thermal type. The quantum type makes use of series excitation of sensor elements due to photons of infrared ray. Changes of resistance and voltage of the sensor element generated by the series excitation are read (photovoltaic type, photoconductive type). The quantum type uses a Narrow gap semiconductor; accordingly, noise caused by thermal fluctuation of electrons has to be suppressed. Furthermore, it necessitates cooling to a liquid nitrogen temperature; accordingly, there is a disadvantage in that a sensor becomes large.
On the other hand, the thermal type converts incident infrared energy into thermal energy to finally read as an electric signal. Accordingly, the thermal type does not necessitate cooling, as a noise countermeasure, to a liquid nitrogen temperature level and is called a non-cooling type. From the viewpoint of miniaturization, lighter weight and lower cost, the non-cooling type is advantageous in comparison with the cooling type. Furthermore, although the quantum type is advantageous in being excellent in the sensor performance (for instance, the detection resolution of temperature (NETD) is <0.1K for the non-cooling type and <0.03K for the cooling type, and the response speed is <10 msec for the non-cooling type and <1 μsec for the cooling type), the non-cooling type as well has been forwarded in the development and an improvement in the performance is remarkable. In order to popularize widely, the magnitude and cost are very important items; accordingly, a camera that mounts a non-cooling type infrared sensor is advantageous. The non-cooling type is applied in surveillance cameras for security and disaster prevention, thermography for nondestructive use and car cameras for securing safe driving during night.
In the thermal type sensors, a PN junction type, a borometer type, a pyroelectric type and a thermo-electromotive (thermopile) type are cited. The thermopile type is constituted of an infrared receiver (infrared ray absorption to heat transformation), a thermopile combining a beam (temperature detection of the receiver) and a reading circuit. For example, see following document:
Document 1: Development of Thermopile type Non-cooling Infrared Detector”, Journal of The Japan Society of Infrared Science and Technologies, Vol. 14(2), p. 44 to 47 (2005).
The thermopile combines a beam and is designed slender and thin.
On the other hand, a device where, in order to lengthen a beam with a magnitude of the device itself maintained constant, a cross sectional shape of the beam is formed in arch is proposed (see such as JP-A No. 11-258039). Furthermore, a device where, in order to cope with torsional stress in an in-plane direction generated in the beam, a projection is disposed at a predetermined position of the beam is proposed (see such as JP-A No. 2007-187495).
However, a slender, long and thin beam like the infrared detector described in the Document 1 and JP-A No. 11-258039, when an infrared receiver or a hollow structure to a beam per se is formed to improve the thermal resolution property, released stress tends to cause more strain. For instance, like FIG. 17B that is an A-A′ sectional diagram of FIG. 17A, a hollow structure 704 is a triangular convex 702 exposed on a (111) plane with a (100) plane of a silicon crystal that forms a silicon substrate 701 as a surface. When beams 705 and 706 are strained, as shown in FIG. 18B, the strained beam might come into contact with a substrate. When the beam comes into contact with the substrate, heat is dissipated from a contact portion 716; accordingly, there is a problem in that an effective length of the beam becomes shorter to deteriorate the detection accuracy.
Furthermore, in JP-A No. 2007-187495, to the torsional stress generated in an in-plane direction, while a contact with the substrate of the beam portion is improved from a line contact with the substrate to a point contact, an amount of heat generated at the infrared receiver is slight; accordingly, even the point contact, upon coming into contact with the substrate, dissipates heat to make the beam length shorter. Still furthermore, the contact with the concave portion of the silicon substrate is not improved.